1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to using a geomagnetic sensor for auto-calibration of a deviation of a magnetic field, and more particularly, to a geomagnetic sensor for auto-calibration of a deviation of a magnetic field using a strength and an azimuth of the magnetic field calibrated using output values detected from separated detection coils and a method of using the same.
2. Description of the Related Art
A geomagnetic sensor measures a strength and a direction of a geomagnetism field. Examples of the geomagnetic sensor include a hole sensor using a hole effect, a superconducting quantum interference device (SQID) sensor using a quantization effect, a fluxgate type sensor using a saturation area of a magnetization curve, and the like.
The fluxgate type sensor is mostly used to measure a geomagnetism, in particular, to measure a geomagnetic field and detect minerals, space, the sea bottom, or the like. The fluxgate type sensor includes a soft magnetic property core formed of a high permeability material, an exciting coil winding around the soft magnetic property core, and a detection coil. The basic detection principle of the fluxgate type sensor uses non-linear magnetic characteristics saturating the soft magnetic property core by generating a magnetic field in the exciting coil using an alternating current and to measure the strength of an external magnetic field by measuring second harmonic voltage components proportional to the external magnetic field. The magnetic field detection method of the fluxgate type sensor includes second harmonic wave detection, pulse position detection, pulse magnitude detection, and so on. The second harmonic wave detection is mainly used.
With the recent development of Micro Electro Mechanical System (MEMS) technology, subminiature fluxgate type sensors consuming a small amount of power can be manufactured. Thus, such a subminiature fluxgate type sensor can be built in various potable electronic devices such as mobile phones, personal digital assistants (PDAs), notebook PCs, or the like.
FIG. 1 is a view illustrating a conventional fluxgate including a separate type exciting coil.
Referring to FIG. 1, the conventional fluxgate includes cores 5, an exciting coil 1 winding around the cores 5, and a detection coil 3 wining around the cores 5. The cores 5 are formed of a high permeability material and may be a single line type core, two parallel cores, a ring type core, or the like. Two parallel cores and the conventional fluxgate including one detection coil 3 are shown in FIG. 1. The exciting coil 1 winds around the cores 5 in the solenoid form and receives an electric drive signal from an external source to excite the cores 5. The detection coil 3 also winds around the cores 5 in the solenoid form and detects an electromotive force from a magnetism generated by driving of the exciting coil 1.
A geomagnetic sensor includes fluxgates to be orthogonal to each other as shown in FIG. 1. The fluxgates correspond to X-axis and Y-axis fluxgates. The geomagnetic sensor calculates a direction and strength of a magnetic field using output values output from the X-axis and Y-axis fluxgates. The strength of the magnetic field in a random position may be obtained by calculating an azimuth. When the azimuth is φ, the azimuth φ is calculated as tan−1(Hy/Hx). Hx denotes the output value output from the X-axis fluxgate, and Hy denotes the output value output from the Y-axis fluxgate.
FIGS. 2A and 2B illustrate the results of detecting a calibrated magnetic field, and of detecting a magnetic field that is not affected by an external magnetic field and the magnetic field that is affected by the external magnetic field.
Referring to FIG. 2A, the strength of a magnetic field measured for X-axis and Y-axis fluxgates orthogonal to each other is calibrated, and the magnetic field forms a complete circle. A geomagnetic sensor system must 360° rotate to obtain the complete circle as shown in FIG. 2A.
Referring to FIG. 2B, the center of a circle obtained by detecting a magnetic field is displaced in a direction of an external magnetic field due to an effect of the external magnetic field, and thus coordinate axes are displaced. A direction of the measured geomagnetism varies depending on the environment. In other words, a geomagnetic sensor is easily affected by a peripheral magnetic field such as buildings, iron bridges, subways, or the like, and an output signal of the geomagnetic sensor greatly varies according to an assembled state, an inclined degree, or the measurement environment of the buildings, the iron bridges, the subways, or the like. In this case, the geomagnetic sensor requires a calibration operation to measure an exact azimuth. If the geomagnetic sensor is used without an appropriate calibration operation, an azimuth of a magnetic field may be changed sharply from Φ to Ψ as shown in FIG. 2B. Although the center of the circle is moved by the affect of the external magnetic field, the movement of the center of the circle may not be calibrated. In this case, the direction of the magnetic field is detected as I not as II. Thus, in a case where the external magnetic field is not continuously calibrated, an azimuth of a geomagnetic field is distorted, and thus the reliability of data is deteriorated. The center of the circle must be moved from point C to point C′ in direction III to calibrate the effect of the external magnetic field. For such a calibration, the geomagnetic sensor system must be rotated to detect output values of X and Y axes of the circle that is affected by the external magnetic field, so as to detect the center of the moved circle.
U.S. Pat. No. 4,953,305 discloses a system including a magnetic sensor and a microprocessor used for a vehicle in which a magnetic field is measured on X and Y axes, and a displacement of coordinate axes of a detected magnetic field exceeding a detection restriction area due to an external magnetic field is automatically calibrated. However, in a calibration method disclosed in U.S. Pat. No. 4,953,305, a slight variation in the direction of a geomagnetism due to a variation in the environment may be calibrated but a severe variation may not be calibrated. The disclosed calibration method varies with each vehicle. Also, before a geomagnetic system is mounted in the vehicle, a variation in the direction of a geomagnetism may not be calibrated. Even after the geomagnetic system is mounted in the vehicle, the vehicle must rotate 360° several times in order to calibrate the variation.
When the vehicle rotates several times to calibrate the variation, a substantial user environment hardly exists in a space for rotating the vehicle. Thus, corrected data must be compared with initial corrected data to determine whether the corrected data is re-corrected, so as to improve a calibration process with a sufficient rotation. Thus, U.S. Pat. No. 5,390,122 discloses a method of operating an auto-calibration system when a system including a geomagnetic sensor moves at a speed of 16 km/h. As disclosed in U.S. Pat. No. 5,390,122, a vehicle is not rotated for calibration, but re-corrected data is compared with initial corrected data. However, data appropriate for calibrating the geomagnetic sensor may not be obtained according to proper circumstances.